linux-kernel - Re: [PATCH v17 02/16] preempt: Track NMI nesting to separate per-CPU counter

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Message-ID: <1a332333-0706-4548-8fbe-d65956f70f99@nvidia.com>
Date: Thu, 5 Feb 2026 17:17:26 -0500
From: Joel Fernandes <joelagnelf@...dia.com>
To: Boqun Feng <boqun@...nel.org>, Peter Zijlstra <peterz@...radead.org>
Cc: Lyude Paul <lyude@...hat.com>, rust-for-linux@...r.kernel.org,
 linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
 Boqun Feng <boqun.feng@...il.com>,
 Daniel Almeida <daniel.almeida@...labora.com>,
 Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
 Gary Guo <gary@...yguo.net>, Björn Roy Baron
 <bjorn3_gh@...tonmail.com>, Benno Lossin <lossin@...nel.org>,
 Andreas Hindborg <a.hindborg@...nel.org>, Alice Ryhl <aliceryhl@...gle.com>,
 Trevor Gross <tmgross@...ch.edu>, Danilo Krummrich <dakr@...nel.org>,
 Andrew Morton <akpm@...ux-foundation.org>, Ingo Molnar <mingo@...hat.com>,
 Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>
Subject: Re: [PATCH v17 02/16] preempt: Track NMI nesting to separate per-CPU
 counter



On 2/5/2026 4:40 PM, Boqun Feng wrote:
> On Wed, Feb 04, 2026 at 12:12:34PM +0100, Peter Zijlstra wrote:
>> On Tue, Feb 03, 2026 at 01:15:21PM +0100, Peter Zijlstra wrote:
>>> But I'm really somewhat sad that 64bit can't do better than this.
>>
>> Here, the below builds and boots (albeit with warnings because printf
>> format crap sucks).
>>
> 
> Thanks! I will drop patch #1 and #2 and use this one (with a commit log
> and some more tests), given it's based on the work of Joel, Lyude and
> me, would the following tags make sense to all of you?
> > Co-developed-by: Joel Fernandes <joelagnelf@...dia.com>

I don't know, I am not a big fan of the alternative patch because it adds a
per-cpu counter anyway if !CONFIG_PREEMPT_LONG [1]. And it is also a much bigger
patch than the one I wrote. Purely from an objective perspective, I would still
want to keep my original patch because it is simple. What is really the
objection to it?

[1]
+#ifndef CONFIG_PREEMPT_LONG
+/*
+ * Any 32bit architecture that still cares about performance should
+ * probably ensure this is near preempt_count.
+ */
+DEFINE_PER_CPU(unsigned int, nmi_nesting);
+#endif

Thanks,
--
Joel Fernandes